A thin film transistor (hereinafter, referred to as a “TFT”) is embedded and used in a liquid crystal display apparatus by using a merit capable of forming the TFT on a large-sized glass substrate and the like. Currently, an amorphous silicon (a-Si) TFT is used in most liquid crystal display apparatuses, but recently, as screen scanning efficiency is increased up to 480 Hz, a TFT having more excellent current driving capacity has been required. An oxide TFT uses metal oxide such as IGZO (In—Ga—Zn—O), ZnO and the like as an active layer, and since the oxide TFT has electron mobility of 10 cm2/Vs or more as illustrated in FIG. 1, the current driving capacity is ten times or higher than an existing a-Si TFT.
However, in the oxide TFT, since a characteristic is changed due to a minute condition change in a manufacturing process, or voltage and light, a depletion-mode operation characteristic having negative threshold voltage VT is shown.
In the existing a-Si TFT, since the threshold voltage VT has a positive value, the a-Si TFT is turned off when gate-source voltage VGS is 0 V and thus a driving circuit is easily configured. However, since the oxide TFT (hereinafter, referred to as an “oxide TFT”) having the depletion-mode operation characteristic have negative threshold voltage VT as illustrated in FIG. 2, the oxide TFT is not turned off and current flows even in the case where the gate-source voltage VGS is 0 V. Accordingly, in order to completely turn off the depletion-mode TFT, the negative threshold voltage VT needs to be applied. In order to solve the problem and configure an embedded circuit of the liquid crystal display apparatus by using the depletion-mode TFT, a new circuit capable of applying the negative threshold voltage VT needs to be designed.
FIG. 3 is a diagram illustrating a DC voltage conversion circuit in the related art, and FIG. 4 is a diagram illustrating a DC voltage conversion circuit in the related art which operates in response to a characteristic of a depletion-mode TFT which improves the DC voltage conversion circuit in the related art (“Charge pump circuit for depletion-mode oxide TFTs”, M. W. Oh, et al., Electronics Letters, Vol. 47, No. 6, 2011).
Referring to FIGS. 3 and 4, since the DC voltage conversion circuit in the related art uses only an N-type TFT, a circuit may be configured by using the a-Si TFT or oxide TFT, and all TFTs are turned off when the gate-source voltage VGS is −5 V, such that the circuit may operate normally even in the depletion-mode TFT. However, when the TFT is turned on, since 0 V is applied as the gate-source voltage VGS, a general TFT having threshold voltage VT of 0 V or more is not turned on, such that the circuit is difficult to operate normally.